Hitherto, it has been widely practiced to use a stretch-oriented film for packaging processed livestock meat, fresh fish meat, raw meat, soup, etc. Particularly, for packaging of frozen food and vacuum packaging livestock meat with bones and fish meat, such a stretch-oriented packaging film is required to have excellent anti-pinhole property and mechanical strength, and a multilayer film including a polyamide resin layer has been generally used therefor.
Particularly, in the case of requiring a relatively thin multilayer film, a multilayer film including a stretched polyamide resin layer has been suitably used.
For production of a multilayer film including a stretched polyamide resin layer, there has been generally practiced a process comprising dry lamination or extrusion lamination of an unstretched polyolefin resin layer onto a stretched polyamide resin layer. However, in the case of necessitating excellent anti-pinhole property and mechanical strength, such a lamination film is caused to have a complicated layer structure, which requires an increased number of production steps and an increased film thickness for ensuring the strength, thus leading to an increased production cost.
For the purpose of reducing the production steps, the multilayer stretching of a co-extruded multilayer film has been commercialized, but this is accompanied with a problem such that the produced film is liable to have an excessively large shrinkability, which makes difficult the secondary processing, such as bag-making, printing or packaging by means of an automatic packaging machine. Particularly, in the case of packing a soup in a hot state into a pouch of such a multilayer film or by automatic packaging with such a multilayer film, the packing becomes difficult due to the film shrinkage.
It has been known that a multilayer film including a stretched polyamide resin layer has an excellent low-temperature strength, but it has been also believed inevitable that the strength is remarkably lowered at a temperature as low as −10° C. for packaging a frozen product.